1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus. More specifically, the present invention relates to a sheet processing apparatus and an image forming apparatus which perform a process of bonding sheets having an image formed thereon together with an adhesive.
2. Description of the Related Art
In recent years, there has been an image forming apparatus, such as a copying machine, a printer, or a multiple function processing device of these, has a sheet processing apparatus which bonds sheets, such as paper, resin films, or metal sheets, having an image formed thereon together. As such method of bonding sheets together, a bonding method by applying a liquid adhesive or a dry-type adhesive has been typically performed.
The needs for adhesive strength for bonding sheets together have recently been diverse. A document for a direct mail whose information showing surface is folded in consideration of confidentiality of information requires adhesive strength to a certain extent so as not to open the folded information showing surface against a load during mailing. When its bonding portion is opened by hand, the adhesive strength needs to have a release force so as not to damage the document.
A sheet to be temporality stuck, such as a tag, needs to have a low adhesive strength and to increase releasability. A confidential letter requires a high adhesive strength so as not to easily release its bonding portion. The adhesive strength and releasability need to be different according to the type and application of sheets to be bonded. Each sheet processing apparatus needs to be adjusted so as to correspond to them.
In order to easily adjust such adhesive strength, there have been disclosed a sheet processing apparatus and an image forming apparatus which have between a sheet and an adhesive an adhesive adjusting layer to which an adhesive strength adjusting agent including a very small hard substance is added, the adhesive adjusting layer adjusting adhesive strength (see Japanese Patent Application Laid-Open Nos. 5- and 7-1872).
There has also been disclosed a method of screen printing an adhesive applying surface in the range of 30 to 60% with inks of four colors using a pressure sensitive adhesive and adjusting an exposed area of the pressure sensitive adhesive to adjust adhesive strength (see Japanese Patent Application Laid-Open No. 7-299974).
The sheet processing apparatus and the image forming apparatus of the related art which have the adhesive adjusting layer between the sheet and the adhesive and adjust adhesive strength require a process of inserting an adhesive adjusting sheet applied with the adhesive strength adjusting agent into between the sheet and the adhesive.
The inserting process is assumed to be performed before printing onto the sheet. Thus a joining portion of the adhesive adjusting sheet and the sheet is required upstream of an image forming portion. The pressure sensitive adhesive is used as an adhesive. A folding and pressing portion needs to be provided downstream of the image forming portion.
When the adhesive adjusting sheet is used, it is set upstream of the joining portion. Each time a region on the sheet requiring adhesive strength adjustment or setting of adhesive strength itself is changed, the strength adjusting sheet of different size needs to be prepared so as to be set to the apparatus. There are many other adjusting matters such as changing an insertion timing in the joining portion of the strength adjusting sheet and the sheet.
There are the problems of upsizing the apparatus, increasing the cost thereof, increasing the unit price of a production sheet, specialization of the adhesive, and reduction in production efficiency (in particular, lack of an on-demand production capacity of small-lot production with a wide variety of products to be made).
In the method of printing the adhesive applying surface in the range of 30 to 60% and adjusting the exposed area of the pressure sensitive adhesive to adjust adhesive strength, printing needs to be performed at least after the adhesive is applied.
Here, the relation between the image forming portion and the adhesive will be considered. When a printing method by contacting a printing plate for typical offset printing, relief printing, intaglio printing, stencil printing, or electrophotograph toner with a printed material is used, the possibility that a printing portion may be consumed due to a viscous substance of the adhesive is high. The use of the liquid-type adhesive as an adhesive can lower the printing properties of ink because the ink is printed on the adhesive.
In the method of adjusting adhesive strength by printing the ink onto the adhesive, the usable adhesive is limited to the dry-type pressure sensitive adhesive. When the pressure sensitive adhesive is used, a pressing portion is necessary to give adhesive properties. Upon pressing by the pressing portion, an offset phenomenon which causes the ink to be transferred to the opposite side of a printing surface can occur.
There are the problems of upsizing the apparatus, increasing the cost thereof, increasing the unit price of a production sheet, and specialization of the adhesive.
The present invention provides a sheet processing apparatus and an image forming apparatus which can adjust adhesive strength by a simple configuration.